It is of course perpendicular to the Horizontal Line also.
6. The Points of Distance. One, called the dis- tance of the picture, is the distance of the eye from the centre of the Perspective Plane. The other called the Vanishing distance, is the distance from the eye to the Vanishing point on the Perspective Plane.
7. The Ground Plane is the table or any flat horizon- tal surface on which the Perspective Plane is supposed to stand.
8. The Base Line is a line necessarily marked across the Ground Plane by the base or bottom of the Per- spective Plane.
fig. 3. plate iu
This figure represents a road with passengers at differ- ent distances on it.
The Base Line is at the bottom of the figure.
The Horizontal Line cuts the four passengers in the middle, and is always parallel to the Base Line.
The Vanishing Point and Centre of the Picture are in this case the same point, where the lines converge on the Horizontal Line.
The Prime Vertical Line is the perpendicular raised on the Base Line, and passing through the Centre of the Picture.
Now to give each figure its relative proportion ac- cording to its distance, draw vanishing lines to the van- ishing point from the extremities of the nearest figure, and in the angle thus formed will be found the heights of all the other figures. To find the height of a figure on any part of the road, only lead off a horizontal line, parallel to the base line, for the foot of the object, and the height is found by raising a perpendicular from the horizontal until it strikes the other converging or van-
